Method and apparatus for avoiding slip in transporting a recording substrate in a fixing station of an electrographic printer or copier

ABSTRACT

In the fixing station, the recording substrate 4 is transported by friction between two rollers (1, 2). In order to tauten the recording substrate (4), it is braked by a braking device (3) upstream of the rollers (1, 2). A length equalizing element (5) equalizes any length differences occurring in the recording substrate (4). The rotational speed of the rollers (1, 2) is controlled as a function of the deflection of the length equalizing element (5). The deflection time (txy) between reaching a deflection point a and a deflection point b is registered and compared with a desired deflection time (tsp). The braking force of the braking device (3) is reduced if the desired deflection time (tsp) is exceeded.

BACKGROUND OF THE INVENTION

The invention relates to a system and a process for avoiding slip duringtransport of a recording substrate in an electrographic printer orcopier. In electrographic printers or copiers, recording substrates incontinuous form are conveyed to various units which are arranged along arecording substrate transport path. In order to ensure secure transportof the recording substrate, recording substrates are often used whichhave an edge perforation. Tractor drives, with the aid of which therecording substrate is able to be transported without slip withinspecific tolerance levels, engage in the edge perforation.

In the region of the fixing device, particular requirement are placed inrelation to slip-free recording substrate transport. In a fixing device,such as is known, for example, from U.S. Pat. No. 4 147 922, therecording substrate is therefore transported by friction between a niproller and a fixing roller. At least one of the two rollers, which arepressed against each other, is driven by a motor for this purpose.During the passage between the two rollers, of which at least one isheated, a toner image loosely applied on the recording substrate isfused into the recording substrate.

In the case of simple transport of the recording substrate throughbetween the fixing roller and the nip roller, the formation of folds inthe recording substrate may occur because of the influence of heat andthe high pressure. Therefore, the recording substrate is tautened beforepassage between the fixing roller and the nip roller. For this purpose,in the recording substrate transport direction, a sliding saddle and apaper brake are provided upstream of the two rollers. The sliding saddleis located between the rollers and the paper brake. The paper brake,which is operated under vacuum, sucks the recording substrate on andbrakes the latter more or less severely according to the availablevacuum. The force in the recording substrate transport direction whichis brought about by the friction between the fixing roller and niproller counteracts the force which is brought about by the paper brake.The recording substrate is tensioned over the saddle.

The recording substrate is led to the paper brake at constant speed overthe recording substrate transport path. In order to equalize temporarydifferences in the recording substrate transport speed, a loop tensioneris arranged upstream of the paper brake.

Such a loop tensioner is known from WO reference 91/09352. The positionof the loop tensioner is scanned by two sensors which are arranged inthe region of the maximum and minimum deflection of the loop tensioner.The rotational speed of the rollers which are transporting the recordingsubstrate and are arranged upstream and downstream of the loop tensioneris controlled as a function of the respective position.

It has now been shown that, in the case of the known devices, slip canoccur between at least one of the fixing roller and the nip roller andthe recording substrate. This slip occurs when the friction between therecording substrate and the saddle and paper brake is greater than thefriction between the recording substrate and the rollers. Thisunfavorable friction behavior has its causes in thermal expansion of thefixing and nip rollers, the oiling up of the fixing roller, uneven nipforces along the fixing and nip rollers and/or in the properties of therecording substrate being processed. Smudging of the printed image oreven its destruction cannot with certainty be excluded.

SUMMARY OF THE INVENTION

The invention is based on the object of indicating a system and aprocess for avoiding slip in the transport of a recording substrate in afixing station of an electrographic printer or copier with the aid ofwhich the occurrence of slip between the recording substrate on the onehand and the fixing roller and/or the nip roller on the other hand canbe counteracted.

This problem is solved using the features specified in patent claims 1and 7. Advantageous refinements and developments of the invention arespecified in the subclaims.

In general terms the present invention is a system for avoiding slip inthe transport of a recording substrate, which is present in continuousform, in a fixing station of an electrographic printer or copier. Therecording substrate is able to be transported by friction in therecording substrate transport direction by means of two rollerscontained in the fixing station. These rollers are preceded by a brakingdevice for tautening the recording substrate in the recording substratetransport direction. The braking device is preceded by a loop tensionerwhich can be pivoted about a pivot axis, for equalizing the length ofthe recording substrate transport path to the length of the recordingsubstrate, which can be fed to the fixing station at constant speed.Sensors are assigned to the loop tensioner at its deflection points. Aroller drive controller controls the rotational speed of the rollers asa function of reaching these deflection points of the loop tensioner. Atime registering module registering the deflection and dwell timebetween reaching a first deflection point, at which there is a maximumlength of the recording substrate transport path, and reaching a seconddeflection point of the loop tensioner. A comparison device compares theregistered deflection and dwell time with a desired deflection and dwelltime. A braking force controller controls the braking force of thebraking device acting on the recording substrate, the control being suchthat the braking force is reduced if the desired deflection and dwelltime is exceeded by the registered deflection and dwell time. Means areprovided which bring about a variation in the basic setting of brakingforce and roller rotational speed when a limiting value of the brakingforce is reached.

Advantageous developments of the present invention are as follows.

The braking force controller controls the braking force such that thebraking force is increased if the desired deflection time (tsp) is notreached.

On reaching a minimum braking force the roller drive controllerincreases the difference between the roller rotational speeds, which areset on reaching various deflection points of the loop tensioner. Thebraking force controller sets the braking force to a desired value.

If a maximum braking force is present and a deflection timecorresponding to the desired deflection time is present, the rollerdrive controller lowers the difference between the roller rotationalspeeds, which are set on reaching various deflection points of the looptensioner. The braking force controller sets the braking force to adesired value.

If a minimum braking force is present and if a maximum differencebetween the roller rotational speeds is present and if a maximumdeflection time is exceeded, the printing operation is interrupted.

A pneumatic brake, which is coupled via a controllable valve to ablower, is provided as braking device. The present invention is also aprocess for avoiding slip in the transport of a recording substrate,which is present in continuous form, in a fixing station of anelectrographic printer or copier. The recording substrate is able to betransported by friction in the recording substrate transport directionby means of two rollers contained in the fixing station. These rollersbeing preceded by a braking device for tautening the recording substratein the recording substrate transport direction. The braking device ispreceded by a loop tensioner, which can be pivoted about a pivot axis,for equalizing the length of the recording substrate transport path tothe length of the recording substrate, which can be fed to the fixingstation at constant speed. Sensors are assigned to the loop tensionersat its deflection points. In a first step the rotational speed of therollers is controlled as a function of reaching these deflection pointsof the loop tensioner. In a second step the deflection and dwell timebetween reaching the deflection point, at which there is a maximumlength of the recording substrate transport path, and reaching anotherdeflection point of the loop tensioner is registered. In a third stepthe registered deflection and dwell time is compared with a desireddeflection and dwell time. In a fourth step the braking force of thebraking device acting on the recording substrate is controlled such thatthe braking force is reduced if the desired deflection and dwell time isexceeded by the registered deflection and dwell time, and a variation inthe basic setting of braking force and roller rotational speed iscarried out when a limiting value of the braking force is reached.

By virtue of the invention, any slip occurring can be detected so earlythat countermeasures can be introduced before impairment to the printedimage can occur. In the case of a change in the paper properties, thenip forces between fixing roller and nip roller because of their thermalexpansion, it is no longer necessary to stop the printer or copier andto undertake manual changes to the nip forces or braking forces. Rather,these forces are automatically adapted to one another in such a way thatany slip occurring can be eliminated without interrupting operations.

According to a refinement and development of the invention, the brakingforce controller controls the braking force such that the braking forceis increased if the desired deflection time is not reached. This ensuresnot only that any possible slip is prevented but, in addition, also thatthe best possible tautening of the recording substrate upstream of thefixing and nip rollers is ensured.

According to a further development and refinement of the invention, onreaching the minimum braking force the difference between the rollerrotational speeds, which are set on reaching various deflection pointsof the length equalizing element, is increased. In addition, the brakingforce controller sets the braking force to a desired value. As a resultof these measures, the setting range is increased such that for the casewhere slip still occurs in spite of a reduction of the braking force toa minimum value, this slip can be counteracted further.

By virtue of a further development and refinement of the invention, if amaximum braking force is present and a deflection time corresponding tothe desired deflection time is present, the difference between theroller rotational speeds, which are set on reaching various deflectionpoints of the length equalizing element, is lowered. Furthermore, thebraking force controller controls the braking force to a desired value.By virtue of these measures, for the case in which there is no slip, thetautening of the recording substrate before its passage through the niproller and fixing roller can be optimized.

According to a further development and refinement of the invention, if aminimum braking force is present, a maximum difference between theroller rotational speeds is present and the maximum deflection time isexceeded, the printing operation is interrupted. By virtue of thismeasure it is ensured that, for the case in which all the settingpossibilities have been exhausted and slip is nevertheless occurring,the printing operation is interrupted and an issue of recordingsubstrates printed with an insufficient printed image is avoided.

According to a further development and refinement of the invention, apneumatic brake, which is coupled via a controllable valve to a blower,is provided as braking device. With the aid of the pneumatic brake, therecording substrate can be sucked more or less strongly onto the slidingsurface of the braking device. By varying the vacuum, the frictionalresistance can be varied. A variation in the vacuum can be undertaken ina simple way by means of a controllable valve.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the present invention which are believed to be novel,are set forth with particularity in the appended claims. The invention,together with further objects and advantages, may best be understood byreference to the following description taken in conjunction with theaccompanying drawings, in the several Figures of which like referencenumerals identify like elements, and in which:

FIG. 1 shows a schematic representation of a fixing station of anelectrographic printer or copier,

FIG. 2 shows a representation of the principle of a length equalizingelement of the fixing station,

FIG. 3 shows a time/speed diagram of the fixing and nip rollers of thefixing station,

FIG. 4 shows a flow diagram for defining the starting conditions for theprocess according to the invention and

FIG. 5 shows a flow diagram for the process according to the inventionfor avoiding slip in the transport of a recording substrate.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An electrographic printer or copier for printing recording substrates 4in the form of continuous paper contains a fixing station, shownschematically in FIG. 1. The fixing station is designed as athermoprinting fixing station. It contains a heated fixing roller 1 anda nip roller 2. The nip roller 2 is pressed in the direction of thefixing roller 1 by a pivoting and pressing device, not shown. The fixingroller 1 comprises an aluminum cylinder with a heat-resistant coatingarranged thereon. The fixing roller 1, which is driven by an electricmotor, is supplied with release agent by an oiling device, not shown.The release agent prevents the adherence of toner particles on thecoating of the fixing roller 1. The drive of the fixing roller 1 and ofthe nip roller 2, comprising aluminum coated with silicone, is carriedout by means of an electric motor drive. This electric motor drive isassigned a roller drive controller 7, which controls the drive inaccordance with the desired roller speed. A recording substrate 4 istransported by friction between the fixing roller 1 and the nip roller2. The transport is carried out in the recording substrate transportdirection 6.

The fixing and nip rollers 1, 2 are preceded in the recording substratetransport direction 6 by a first and a second preheating saddle 11, 12.With the aid of the preheating saddles 11, 12, the continuous paper 4 ispreheated for the actual fixing process between the fixing and niprollers 1, 2. For this purpose, the continuous paper 4 slides flat onthe sliding surfaces of the preheating saddles 11, 12 which face thecontinuous paper 4. In order to optimize the contact between the slidingsurfaces and the continuous paper 4, the preheating saddles 11, 12 havesuction openings, not shown, which are connected via hose connections 13and a braking device 3, positioned upstream of the preheating saddles11, 12 in the recording substrate transport direction 6, to a blower 14producing a vacuum. The braking device 3 is coupled to the blower 14 viaa valve 15. The valve 15 can be controlled by a braking force controller10 in such a way that a desired vacuum can be set in the braking device3. By virtue of the interaction of the braking device 3 with the fixingand nip rollers 1, 2, two opposed forces, namely a driving forcedirected in the recording substrate transport direction 6 and a brakingforce directed counter to the recording substrate transport direction 6,act on the continuous paper 4. In consequence, the continuous paper 4 isdrawn free of folds over the preheating saddles 11, 12.

By means of a tractor drive 16, the continuous paper is fed at constantspeed VA (see FIG. 3) to the region between braking device 3 and therollers 1, 2. In order to counteract speed fluctuations in the fixingstation, and to avoid negative effects on the recording substrate 4, alength equalizing element 5 is provided upstream of the braking device 3in the recording substrate transport direction 6. The length equalizingelement 5 is designed as a loop tensioner 5. The loop tensioner 5 ismounted so that it can pivot about a pivot axis 17. From this pivot axis17, a guiding metal sheet 18 bent over at its outer end extends in therecording substrate transport direction 6. The guiding metal sheet 18 ispressed outward by a spring, not shown, such that the recordingsubstrate transport path is lengthened. The loop tensioner 5 can bepivoted between two end positions, see FIG. 2. At the end position inwhich the recording substrate transport path has its maximum length, afirst sensor a is arranged. A second sensor b is arranged in thepivoting position of the loop tensioner 5 in which the recordingsubstrate transport path reaches its minimum length. The sensors a, bare coupled electrically to a time registering module 8. This timeregistering module 8 registers both the reaching of the loop tensionerof a sensor a, b and also the time which elapses between reaching thefirst sensor a and reaching the second sensor b and vice versa. The timeregistering module 8 is coupled to the roller drive controller 7, acomparison device 9 and the braking force controller 10 in such a waythat information may be exchanged between these units.

According to FIG. 3, the paper is conveyed by the tractor drive 16 atconstant speed VA to the fixing station. There, it is gripped by thefixing roller 1 and the nip roller 2 and conveyed by friction in therecording substrate transport direction 6. In so doing, it is tensionedbetween the braking device 3 and the rollers 1, 2 over the preheatingsaddles 11, 12. The loop in the continuous paper 4 upstream of thebraking device 3 ensures secure functioning of the fixing device andprotection of the continuous paper 4 against damage. The rotationalspeed of the fixing and nip rollers 1, 2 is controlled as a function ofthe loop size. If there is a large loop, the paper transport in thefixing station is, for example, accelerated to a speed VO1. If there isa small loop, the paper transport in the fixing station is deceleratedto a lower speed VU1. The upper paper speed VO1 is greater than theaverage speed VA, and the lower paper speed VU1 is lower than theaverage paper speed VA.

The loop size is registered by the two sensors a, b of the looptensioner 5. The presence of the large loop is reported by the firstsensor a and the presence of the small loop is reported by the secondsensor b to the time registering module 8. In regular slip-freeoperation, the loop information obtained is forwarded to the rollerdrive controller 7, which changes the rotational speed of the fixing andnip rollers 1, 2 accordingly. In addition, the time registering module 8determines the time intervals between reaching the first sensor a andreaching the second sensor b, and vice versa. These times t11, t12depend on the speed of the continuous paper 4 and are constant in thenormal case. If slip is now effected between fixing and/or nip rollers1, 2 and the continuous paper 4, the time increases for reducing theloop. This is the time during which the loop tensioner 5 moves from thefirst sensor a to the second sensor b. The lengthening of this time t12is in this case directly proportional to the slip. This time t12 istherefore suitable as a measure of the slip.

Since the time t12 is a measure of the slip, it is reported by the timeregistering module 8 to the comparison device 9. In the comparisondevice 9, the loop reduction time t12 is compared with a desired timetsp. Depending on a result this comparison, measures which counteractslip are introduced using the roller drive controller 7, the brakingforce controller 10 and the comparison device 9, which are all coupledto one another. These measures include a variation of the range offluctuation of the paper speed v in the fixing station and a variationof the braking force of the braking device 3. The braking force can bevaried by varying the vacuum p in the braking device 3. The vacuum p canbe set by the valve 15.

Since it is possible to process different recording substrates 4 using aprinter or copier, starting conditions for the transport of therecording substrate 4 in the fixing station must be defined ordetermined before beginning the printing or copying process. Theproperties of the recording substrate 4 include its width pbr, its paperweight, its surface condition, electrostatic properties, anypre-printing on the recording substrate 4, etc. According to FIG. 4, apressure change value dp for the braking device 3, a speed change valuedv, a maximum paper speed vmax, a minimum pressure pmin and the paperwidth pbr are prescribed. The speed change value dv and the pressurechange value dp specify the increment by which speed v and vacuum p maybe changed during the process. As a function of the paper width pbr, amaximum pressure pmax and a starting pressure pstart are determined. Ina further step, a minimum speed vmin is defined as a function of thepaper width pbr. If the paper width is smaller than a first limitingvalue for the paper width pbr 1, a minimum speed of vmin=v1 is then set.If the paper width pbr is greater than the first limiting value of thepaper width pbr 1 and smaller than or equal to a maximum paper width pbrmax, a minimum speed vmin=v2 is then set. In this case, it is true that

    v2=v1+dv.

In a further step, the starting speed vstart is determined as a functionof the paper width pbr. If the paper width pbr is greater than the firstlimiting value of the paper width pbr 1 and smaller than or equal to themaximum paper width pbr max, a starting speed vstart=v1 is then set. Ifthe paper width is greater than or equal to a minimum paper width pbrmin but smaller than a second limiting value of the paper width pbr 2, astarting speed vstart=v3 is set. If neither of the first-mentionedconditions applies, a starting speed vstart=v2 is set. In this case, itis true that

    v3=v1+2·dv.

The process for avoiding slip is represented in the flow diagramaccording to FIG. 5. Initially, a time variable t=0 is set. The timeregistering module 8 supplies the time interval t×2, which specifies thetime which the loop tensioner 5 needs for a movement from the firstsensor a to the second sensor b. The time interval t×2 depends on theconstant speed vA and the difference between the upper speed VO1, VO2,VO3 and the lower speed VU1, VU2, VU3. Here, it is true that V1=(VU1,VO1), V2=(VU2, VO2), V3=(VU3, VO3).

The time interval t×2 is compared with a desired time tsp. The desiredtime tsp is constant for all the possible speeds v1, v2, v3. If,however, the speed change value dv is selected to be large, it must thenbe varied in accordance with the set speed. The desired time tsp isincreased if the upper paper speed VO1, VO2, VO3 is reduced, and isreduced if the upper speed VO1, VO2, VO3 is increased. The permissibleslip can thereby be kept constant in spite of a great range offluctuation of the speed v.

If the time interval t×2 is greater than the desired time tsp, thevacuum p is reduced by the pressure change value dp. A check is thenmade as to whether the vacuum p is greater than or equal to the minimumpressure pmin. If this is the case, the time variable is then set to t=0and a further comparison of the time interval t×2 with the desired timetsp is carried out. If the time interval t×2 is still larger than thedesired time tsp, the vacuum p is reduced further until it is less thanthe minimum pressure pmin.

If this is the case, the vacuum p is set once more to the value of theminimum pressure pmin and the paper speed is increased by the speedchange value dv. If the paper speed v following its change is less thanor equal to the maximum paper speed vmax, the vacuum p is then set tothe value of the starting pressure pstart and the control process isbegun once more following setting of the time variable to t=0. If thevalue of the paper speed v following its increase is greater than themaximum paper speed vmax, the paper speed v=vmax is set. A check is thenmade as to whether the time interval t×2 is greater than a maximum valuetmax for the time interval t×2. In this case, it is true that

    tmax>tsp.

If the time interval t×2 is greater than the maximum desired time tmax,the printing operation is interrupted and a fault message F is output onthe printer or copier. If the time interval t×2 is, however, not greaterthan the maximum desired time tmax, the printing operation ismaintained.

If the time interval t×2 is smaller than the desired time tsp, thetautening of the continuous paper 4 is optimized. If the condition thatthe time interval t×2 is smaller than the desired time tsp is alwaysfulfilled during a comparison time dt, and if subsequently thecomparison time dt is exceeded, the vacuum p is increased by thepressure change value dp. If, following this pressure increase, thevalue of the vacuum is less than or equal to the maximum pressure pmax,the time variable is then set to t=0 and the control process is startedanew. If, on the other hand, the value of the vacuum p exceeds themaximum pressure pmax, the paper speed v is reduced by the speed changevalue dv. If the value of the paper speed v following its reduction isgreater than or equal to the minimum paper speed vmin, the vacuum p isthen set to the starting pressure pstart and the control process isstarted anew. If the value of the paper speed v is smaller than theminimum speed vmin, the paper speed v is then set to the value of theminimum speed vmin and the control process is started anew. If thereforethe maximum pressure pmax and the minimum speed vmin have been reachedand the time interval t×2 is smaller than the desired time tsp, no morechanges are made, since slip is no longer occurring.

The invention is not limited to the particular details of the method andapparatus depicted and other modifications and applications arecontemplated. Certain other changes may be made in the above describedmethod and apparatus without departing from the true spirit and scope ofthe invention herein involved. It is intended, therefore, that thesubject matter in the above depiction shall be interpreted asillustrative and not in a limiting sense.

What is claimed is:
 1. A system for avoiding slip during transport of arecording substrate in a fixing station of an electrographic printer orcopier, the recording substrate having a continuous form, comprising:therecording substrate being transportable by friction in a recordingsubstrate transport direction by two rollers contained in the fixingstation; said rollers being preceded by a braking device for tauteningthe recording substrate in the recording substrate transport direction;the braking device being preceded by a loop tensioner which is pivotableabout a pivot axis for equalizing the length of the recording substratetransport path to the length of the recording substrate, which isfeedable to the fixing station at constant speed; sensors respectivelyassigned to the loop tensioner at first and second deflection pointsthereof; a roller drive controller controlling a rotational speed of therollers as a function of reaching said first and second deflectionpoints of the loop tensioner; a time registering module registering adeflection and dwell time between reaching the first deflection point,at which there is a maximum length of the recording substrate transportpath, and reaching the second deflection point of the loop tensioner; acomparison device comparing the registered deflection and dwell timewith a desired deflection and dwell time; a braking force controllercontrolling a braking force of the braking device acting on therecording substrate, the control being such that the braking force isreduced if the desired deflection and dwell time is exceeded by theregistered deflection and dwell time; and device for varying a basicsetting of braking force and roller rotational speed when a limitingvalue of the braking force is reached.
 2. The system as claimed in claim1, wherein the braking force controller controls the braking force suchthat the braking force is increased if the desired deflection time isnot reached.
 3. The system as claimed in claim 1, wherein, on reaching aminimum braking force:the roller drive controller increases a differencebetween roller rotational speeds of the rollers, the speeds being set onreaching various deflection points of the loop tensioner; and thebraking force controller sets the braking force to a desired value. 4.The system as claimed in claim 1, wherein, if a maximum braking force ispresent and a deflection time corresponding to the desired deflectiontime is present:the roller drive controller lowers a difference betweenroller rotational speeds of the rollers, the speeds being set onreaching various deflection points of the loop tensioner; and thebraking force controller sets the braking force to a desired value. 5.The system as claimed in claim 1, wherein, if a minimum braking force ispresent, and if a maximum difference between the roller rotationalspeeds is present, and if a maximum deflection time is exceeded, theprinting operation is interrupted.
 6. The system as claimed in claim 1,wherein the system further comprises a pneumatic brake, which is coupledvia a controllable valve to a blower, as braking device.
 7. A processfor avoiding slip in transporting a recording substrate in a fixingstation of an electrographic printer or copier, the recording substratehaving a continuous form, comprising the steps of:providing that therecording substrate is transportable by friction in a recordingsubstrate transport direction by two rollers contained in the fixingstation; said rollers being preceded by a braking device for tauteningthe recording substrate in the recording substrate transport direction,and the braking device being preceded by a loop tensioner, which ispivotable about a pivot axis, for equalizing a length of the recordingsubstrate transport path to a length of the recording substrate, whichis feedable to the fixing station at constant speed; and sensors beingassigned to the loop tensioner and respectively located adjacent atleast first and second deflection points thereof; controlling therotational speed of the rollers as a function of reaching said first andsecond deflection points of the loop tensioner; registering a deflectionand dwell time between reaching the first deflection point, at whichthere is a maximum length of the recording substrate transport path, andreaching the second deflection point of the loop tensioner; comparingthe registered deflection and dwell time to a desired deflection anddwell time; controlling the braking force of the braking device actingon the recording substrate such that the braking force is reduced if thedesired deflection and dwell time is exceeded by the registereddeflection and dwell time, and a variation in the basic setting ofbraking force and roller rotational speed is carried out when a limitingvalue of the braking force is reached.